1. Field of the Invention
The present invention relates generally to an improved actuator design for applying a varying force on an actuator stem during actuation. More particularly, the present invention relates to an improved actuator design incorporating a magnetic actuating force for use in machines, such as surface mount placement equipment, die and attach equipment, tape and reel production equipment and the like.
2. Description of the Invention Background
Solid state electrical devices are typically produced by attaching a plurality of miniaturized electrical components to a common substrate, such as a printed circuit board. The proper performance of the device is dependent upon the precise placement of each electrical component on the substrate. Recent advances in the miniaturization of electronics have brought about the need for component placement capabilities with accuracies within a few hundred microns. The difficulty in placing electrical components to that level of precision limits the achievable miniaturization of the device, which is directly related to the operational speed of the device, and the variability in the performance characteristics of the device.
The placement of electrical components is commonly performed using a pick and place machine to surface mount the electrical components on the substrate. Pick and place machines typically employ a number of movable heads each having a nozzle through which a vacuum or suction is applied to pick a component off a continuous feed assembly, such as a tape and reel assembly. A computer in the pick and place machine is used to control the movement of the head and the machine precisely places the component on the substrate and the vacuum or suction is then released.
The distance travelled by the head during the picking and placing operations is controlled either electronically or is set to a predetermined distance. In electronically controlled heads, the head is driven by a specially dedicated motor and the precise distance between the head and the location where the component is to be picked up or placed is encoded into the computer and the movement of the head is precisely incremented to perform the pick and place operation. In pick operations using a spring actuated head, the computer aligns and lowers the head to a preset level without taking into account the size of the component. When the nozzle contacts the component prior to reaching the preset level, continued lowering of the nozzle, known as overstroke, compresses the spring so as to avoid crushing the component or damaging the nozzle. The force exerted by the spring also ensures good contact between the nozzle and the component so that a sufficient vacuum or suction can be applied to lift the component off the assembly. Once the preset level is reached, the machine then lifts the head and the component off the feed assembly. The placement of the component using the spring actuated head occurs in an analogous fashion.
A problem with spring actuated heads is that, after contact is made between the nozzle and the component, there is a period of time and overstroke required to compress the spring and build a sufficient force to produce good contact for applying the vacuum. Another problem is that the continued application of force during any overstroke of the head can damage the component. The potential for damage to a component is an increasing concern with electronically controlled heads as the heads must brought into more firm contact with the larger components to enable a sufficiently strong vacuum to be formed to pick up the larger components. The precision required in the electronic head does not provide a large tolerance for variations in component sizes unless a sensor feedback system is employed that will tend to further slow down the pick and place process.
In addition, while spring actuated heads provide control over the placement of small articles, such as those less than 10 mm.times.10 mm, comparable to the electronically controlled heads, the precision of the spring actuated heads diminishes when used with larger articles. We have found that when the spring actuated heads are used to place larger components (i.e. greater than 10 mm.times.10 mm), the compressive and expansive forces exerted by the components on the spring are transferred as helical forces on the attached nozzle. The helical forces generated by the spring skews the orientation of the nozzle such that larger components can not be placed using spring actuated heads with the same precision achievable with smaller components. The inability to precisely place larger components with the spring actuated heads has resulted in a dramatic reduction in the overall capacity of pick and place machines because only the electronically controlled heads can be used to reliably place large components, such as 20-lead small outline dual in packages. An additional problem with spring actuators is the need for spring hangers in the heads, which can cause additional maintenance problems and increased machine downtime.
Advances in semiconductor technology have resulted in the development of larger, and more costly components to be used in electrical devices and, as such, have placed increasing demands on the capabilities of manufacturers to produce large component devices in a cost effective manner. The primary options that manufacturers have to increase their capacity are to upgrade their present equipment to replace all of the spring actuated pick and place heads with electronically controlled heads or to purchase new pick and place equipment which consist entirely of electronically controlled heads. Either of these options represent significant expenditures and possibly a significant loss due to the premature retiring of existing machinery, both of which will greatly increase the overall production cost of the devices and will do little to increase the yield by reducing the extent of damage to the components during pick and place operations. Many of the pick and place machines employing spring actuators are currently used by industry to place larger components; however, the full capacities of the machines cannot be exploited due to the problems with the spring actuated heads. Also, various die and attach equipment and tape and reel production machines suffer the same problems with spring actuated placement heads. As such, there is great need to increase the speed and performance of machines for the placement of components.
The present invention is directed to an improved actuator design which overcomes, among others, the above-discussed problems so as to allow machines to more quickly and accurately place components.